Submarine Super 99 (2003)

In the year 20XX AD, mysterious shipwrecks have been continuing to occur near Japan. Oki Juzo and Itsuro disappears to investigate these incidents. Just before they left, they came by a key engraved with the letters "SS-99" in the underground lab where Juzou's grandchild and Itsuro's younger brother, a senior high student Oki Shinji, had been improving various "99 Shikipo Heichu" rifles. As the two progresses in their investigation, the two get caught when they sneak into a laboratory and are lead to a workshop underground. There, one never seen before huge submarine rests.